The piloting of any aircraft requires the knowledge of its relative speed in relation to the air, that is to say to the relative wind. This speed is determined using sensors sensing the static pressure Ps, the total pressure Pt, the angle of attack α and the sideslip angle β. α and β give the direction of the speed vector in a reference system, or reference base, linked to the aircraft and Pt-Ps gives information linked to the modulus of this speed vector. In fact Pt-Ps gives so-called conventional airspeed information, critical for piloting, because it indicates whether or not the aircraft is present in a safe flight domain. The four aerodynamic parameters therefore make it possible to determine the speed vector of an airplane and, incidentally, of an aircraft with rotor, called helicopter, and with tilting rotor, called convertible.
The measurement of the total pressure Pt is usually done by measuring the stagnation pressure of the flow using a so-called Pitot tube. This is a tube that is open at one of its ends, and blocked at the other. The open end of the tube substantially faces the flow.
Inside the Pitot tube, close to the blocked end, there is an orifice connected to a means for measuring the air pressure prevailing therein. The thread of air penetrating into the tube through the open end of the tube is slowed down to zero speed in the tube. The slowing down of the speed of the air tends to increase the pressure of the air. This increased pressure forms the total pressure Pt of the air flow.
In practice, the air flow may convey liquids or solid particles likely to penetrate into the Pitot tube and to build up in the tube at the blocked end. To avoid having a build-up of liquid disrupt the pressure measurement, there is generally provided, at the blocked end, a drain hole through which any liquids can be evacuated.
In this hole both the solid particles and a portion of the air which has entered into the Pitot tube also circulate. Thus, the slowing down of the air in the tube is not complete and the measurement of total pressure Pt is corrupted. More specifically, the greater the effort to avoid the build-up of large particles or of a quantity of liquid, the more the total pressure measurement is corrupted by increasing the dimensions of the drain hole. Conversely, the greater the effort to improve the measurement of total pressure Pt by reducing the dimensions of the drain hole, the greater the risk of build-up of solid particles or of congestion by the liquid. With a Pitot tube, there therefore has to be a trade-off between quality of the measurement of total pressure Pt and risk of disruption of the measurement because of particles conveyed by the air flow where the measurement is performed.
The measurement of the static pressure Ps is usually done by means of cavities that open into the flow through an orifice situated substantially at right angles to the flow. In the cavity, there is an orifice connected to a means for measuring the air pressure prevailing therein. This pressure forms the static pressure Ps of the flow. Some Pitot tubes can be equipped with static pressure taps positioned on their sides. This type of aerodynamic probe is also called Pitot-static probe.
Currently, if the drain holes of the probes installed on aircraft are blocked, or if particles of sand or of volcanic ash are present in the tube itself, the probe is removed, then cleaned with soapy water. The change of probe entails checking the seal-tightness of the pneumatic subsystem, which renders the operation relatively lengthy, of the order of thirty to fifty minutes for an aircraft.